1. Field of the Invention
The present invention relates to an optical transmitter provided with an optical modulator of, for example, Mach-Zehnder type, and a control method of the optical transmitter. More specifically, the present invention relates to an optical transmitter which is capable of effectively maintaining an extinction ratio of an optical modulator and improves the extinction ratio under given conditions, and a control method of the optical transmitter.
2. Description of the Related Art
As for an optical transmitter used for an optical communication system, a direct modulation method that modulates a drive current to be supplied to a semiconductor laser in accordance with a data signal is known.
However, if the direct modulation method is used, there is the tendency that the influences of dynamic wavelength variations (chirping) in the optical output signal of the optical transmitter become increasingly large due to the influences of the wavelength dispersion in the optical fiber as the transmission rate becomes high. Apparently, the direct modulation method is not suitable for long-distance transmission.
To eliminate the problem, the study has been carried out on the use of an optical transmitter provided with a Mach-Zehnder optical modulator which does not easily cause the chirping theoretically.
Even if there is damping of the transmission signal in the long-distance transmission, the maintenance of the extinction ratio is essential to the optical transmitter using the Mach-Zehnder optical modulator in order to attain the signal transmission with a predetermined level of precision.
FIG. 1 shows the composition of a conventional optical transmitter including a Mach-Zehnder optical modulator.
As shown in FIG. 1, a Mach-Zehnder optical modulator 20 receives the direct-current light from a LD (laser diode) light source 10. A driver circuit 40 drives the optical modulator 20 in response to the electric input signal which is externally supplied to the driver circuit 40, so that the optical modulator 20 transmits the optical output signal according to the input signal concerned.
FIG. 2 shows the composition of the Mach-Zehnder optical modulator in FIG. 1. FIG. 3 is a diagram for explaining the input/output characteristics of a Mach-Zehnder optical modulator.
As shown in FIG. 2, the Mach-Zehnder optical modulator is configured so that the input light from the light source is split into two light beams on the two optical waveguide paths. The voltage as the input signal is applied to the electrode on one of the two optical waveguide paths, and the Mach-Zehnder optical modulator outputs the optical output signal. The optical output signal which is output by the optical modulator is in accordance with the modulation-characteristic curve as shown in FIG. 3.
When the high-speed input signal is supplied as the applied voltage of the optical modulator, the high-speed optical output signal is obtained at the output of the optical modulator.
Hereinafter, the difference in voltage between the top (peak) and the bottom (valley) in the waveform of the input signal to the Mach-Zehnder optical modulator which is capable of using a maximum range of the modulation characteristic curve will be referred to as Vπ (FIG. 3).
As indicated in FIG. 3, the extinction ratio EX mentioned above is represented by a value derived from the ratio of the maximum value VH to the minimum value VL in the waveform of the optical output signal of the optical modulator. Namely, the extinction ratio EX is represented by the following formula.EX=10×log VH/VL[dB]
When the value of the extinction ratio EX becomes poor (small), it indicates that the optical-transmission characteristics will deteriorate. Moreover, the extinction ratio EX tends to become poor (small) due to the inter-symbol interference when the bit rate becomes large.